In chemistry, the sulfonamide functional group (also spelt sulphonamide) is -S(=O)2-NH2, a sulfonyl group connected to an amine group. Relatively speaking this group is unreactive. The amine center is no longer basic. The S-N bond is cleaved only with difficulty. Because of the rigidity of the functional group, sulfonamides are typically crystalline. For this reason, the formation of a sulfonamide is a classic method to convert an amine into a crystalline derivative which can be identified by its melting point. Many important drugs contain the sulfonamide group.A sulfonamide (compound) is a compound that contains this group. The general formula is RSO2NH2, where R is some organic group. For example, "methanesulfonamide" is CH3SO2NH2. Any sulfonamide can be considered as derived from a sulfonic acid by replacing a hydroxyl group with an amine group.In medicine, the term "sulfonamide" is sometimes used as a synonym for sulfa drug, a derivative or variation of sulfanilamide.